Single photon source based on transmitters with selectively distributed frequencies

ABSTRACT

Apparatus for single photon source based on emitters with frequencies distributed in a chosen manner. In one embodiment, the apparatus comprises an opto-electronic component capable or emitting light pulses containing a single photon comprising a resonant optical cavity and a group of photon emitters placed in this optical cavity, a single one of these emitters having an emission frequency equal to approximately the resonant frequency of the cavity characterised in that all emitters have a spectral distribution with a concentration of emitter frequencies at a given frequency, and in that the cavity is made so that its resonant frequency is different from this concentration frequency so that the number of emitters with an emission frequency corresponding to the resonant frequency of the cavity is close to one.

The present patent application is a non-provisional application ofInternational Application No. PCT/FR01/01637, filed May 28, 2001.

FIELD

The invention relates to opto-electronic components capable ofdeterministically emitting light pulses containing one and only onephoton.

The main applications of this component are in metrology and in securedtelecommunications.

BACKGROUND

A “single photon source” is a component or a system capable ofgenerating light pulses containing one and only one photon. A highperformance single photon solid source potentially has many importantapplications, for example in metrology (light flux or energy standard)or in telecommunications, by enabling absolute security of exchangesusing quantum cryptography.

Note that in the telecommunications field, the envisaged applicationsfor quantum cryptography mainly concern emission on optical fibres atusual wavelengths (1.3-1.55 μm), but also by telecommunications in freespace (Earth—satellite or inter-satellite communications, submarinecommunications within the blue-green emission window, short distanceland communications).

For this application to quantum cryptography, it is essential thatpulses output by the source never contain more than one photon toachieve unconditional confidentiality. At the moment, very attenuatedlaser pulses containing 0.1 photon on average, and which are only veryimperfect approximations of single photon pulses, are used foravailability reasons.

These sources have been used to validate the compatibility of quantumcommunication protocols with the existing optical network, but a Poissondistribution of their pulses causes severe limitations for real use;firstly, 90% of pulses do not contain any photons, which severely limitsthe transmitted data rate; a more severe problem is that security iscompromised by the fact that 1% of pulses (about 10% of useful pulses)contain more than one photon.

Therefore, a spy could intercept the entire transmitted sequence, andcould analyse and retransmit the pulses containing several photonswithout errors. The only action taken by this spy would be to reduce thetransmitted rate rather than increase the error rate, and it would beimpossible to distinguish his action from the presence of optical losseson the line. Therefore it is important to develop a source capable ofdelivering pulses that never contain more than one photon, with thehighest possible probability of containing one photon.

If single atoms in cavities are capable of producing such a source, thesophistication of the experimental system necessary to prepare, storeand manipulate these atoms makes it impossible to consider theirapplication on a large scale. Therefore the deployment of quantumcryptography depends on the development of a low cost high performancesolid source of single photons.

A single photon source uses a single emitter (atom, ion, molecule, etc.)to guarantee that the system only stores one elementary electrical oroptical excitation, and becomes de-excited later by emitting not morethan one photon.

The emission of this localized single emitter is naturallyomnidirectional; therefore it needs to be placed in an opticalmicrocavity in order to efficiently collect the emitted radiation, andfor example to be able to inject it into an optical fibre. The idealsituation is a situation in which the emission is concentrated in asingle mode of the cavity, such that all single photons can be preparedin the same quantum state.

In the past, this combination of a single emitter and a monomodemicrocavity has not been made successfully, since there are a number ofproblems:

-   -   the atoms, ions or molecules are very imperfect emitters.        Molecules have a short life, limited by optical tarnishing or        “photodarkening”; the radiation efficiency of atoms and ions        placed in a solid matrix is frequently low, and it is difficult        to control their numbers;    -   at the moment, there is no genuinely monomode monolithic optical        microcavity. In 1987, Yablonovitch proposed to start from an        artificial crystal with a prohibited photonic band (that does        not support any electromagnetic mode within a given frequency        range), and introduce a single electromagnetic mode in the        prohibited band in a controlled manner, by introducing an        appropriate defect in the crystal. Although genuine progress had        been made in 98-99 in the domain of making crystals with a        prohibited three-dimensional photonic band, this approach is        found to be technologically very difficult to implement.

Several approaches have been proposed to solve one of these problems.

It is tempting to replace the usual atom, ion or molecule emitter by asemiconducting emitter, a quantum well or a quantum box that has aradiation efficiency of close to one, and which might be electricallypumpable.

Yamamoto et al. has thus proposed to use a quantum well as the activemedium (1), since the electronic states of a well are not discrete, itis then necessary to inject a single electron-hole pair at a time if itis required to obtain a single photon. This result may be obtained usingCoulomb blockage to inject exactly one electron and one hole in thequantum well. However, in this approach it appears very difficult toincrease the operating temperature above 0.1 K, which strongly reducesits usefulness. J. M. Gérard and B. Gayral also proposed to use a singlesemiconducting quantum box as an emitting centre ([2]). The quantumboxes obtained by auto-organized growth have many advantages in thiscontext, when they are compared with the most frequently studied atoms,ions or molecules; these advantages include good stability, radiationefficiency very close to one while the heat emission of carriers isnegligible (in other words T<150K for the most frequently used InAsquantum boxes emitting at close to 1 μm) and the possibility of nonresonant electrical or optical pumping due to the efficient capture ofcarriers injected in the barrier.

As for the quantum well, there is apparently nothing to prevent theinjection of several electron-hole pairs into the quantum box at a time,and observing the emission of several photons.

It is observed experimentally that these photons are emitted atdifferent wavelengths corresponding to different charge states of thequantum box, due to the strong Coulomb interaction between trappedcarriers. Therefore, it is sufficient to spectrally filter the emissionfrom a quantum box to observe the emission of a single photon in a wellchosen spectral window, after impulse pumping.

J. M. Gérard and B. Gayral also proposed to use the “Purcell effect”(exaltation of the spontaneous emission rate of an emitter in a cavity),to very preferentially collect photons useful in a given mode.

Note that the usual monolithic microcavities (micro-column, micro-disk,micro-sphere, etc.) support a discrete set of confined optical modes anda continuum of leakage modes.

When a monochromatic emitter is put in resonance with a confined mode ofthe microcavity, a very strong exaltation of the spontaneous emission ofthe emitter towards this mode is observed under some conditions. In thecase of InAs quantum boxes, the inventors observed an emission towardsthis single confined mode 17 times faster than towards all leakage modesfrom the microcavity ([3]). This effect makes it possible to couple ofthe order of 95% (=17/(17+1)) of photons towards the resonant mode ofthe microcavity.

The inventors thus recently proposed to make a single photon source byputting a single quantum box (for example made of InGaAs) inside anoptical microcavity (for example an GaAs/AlAs micro-column) andresonance with a confined mode of this microcavity (the fundamental modein this case) as shown diagrammatically in FIG. 1 which represents anGaAs/AlAs micro-column, in other words a set containing a microcavity“λ” (in other words a microcavity with a thickness equal to one opticalwavelength) made of GaAs, sandwiched between two Bragg mirrors, eachcomposed of an alternating stack of quarter wave layers of GaAs andAlAs. An InGaAs quantum box is placed in this cavity, in resonance withthe cavity mode. This micro-column is placed on a GaAs substrate. Thediameter of this micro-column is 1 μm.

This association of a quantum box/microcavity can simultaneously achievethe emission of single photons and highly preferential coupling with agiven mode, according to the operating method described herein.

Impulse optical pumping of the GaAs barrier is achieved; thephotogenerated carriers are very quickly captured by the quantum box (orby the free surfaces of the column). The power of the pump is adjustedso as to inject on average of the order of five electron-hole pairs perpulse into the quantum box, such that the probability of having at leastone electron-hole pair in the quantum box is very close to 1.

After recombining the excess pairs, with emission of photons at energiesshifted from the cavity mode by the Coulomb interaction betweencarriers, there is still one pair in the quantum box which is then inresonance with the mode.

Due to the Purcell effect, this final photon is emitted verypreferentially in the fundamental mode of the micro-column. Therefore,the system proposed here ideally acts like a converter of Poisson pulsesfrom a pump into a stream of single photon pulses. This converter may beintegrated vertically and monolithically with an emission laser throughthe surface in order to form a micro-source of electrically pumpedsingle photons. An operating temperature equal to at least 77K can beobtained.

It is difficult to isolate a single box in a microcavity. Auto-organizedgrowth techniques usually lead to the manufacture of dense quantum boxassemblies (typically 400 boxes per μm² for InAs/GaAs). The size ofthese quantum boxes fluctuates; each box has a spectrally very fineemission ray, but their emission wavelength is distributed at randomover a wide spectral range (30 to 100 meV in the case of InAs boxes inGaAs). A typical microcavity has a section of a few μm², and thereforecontains of the order of 1 000 quantum boxes; at the centre of thedistribution, there will be of the order of 10 quantum boxes in thecavity for a spectral range of 1 meV. Therefore it is necessary tostrongly reduce the number of quantum boxes in the cavity if it isimportant to be sure that there is a single box in resonance with theuseful mode. Several solutions are then possible:

-   -   a technological approach could be used to reduce the number of        boxes (for example the plane of the boxes could be structured        before defining the microcavity); all imaginable approaches of        this type are very cumbersome, and tend to degrade the quality        of the optical microcavity;    -   in 1999 (ref. [5]), it was proposed to start from a very sparse        boxes plane (about 10 boxes per μm²). Spectrally, there is then        typically one quantum box every 5 meV, and the probability of        observing two quantum boxes with the same emission wavelength is        very low. The temperature can then be adjusted (of the order of        20K in practice) so as to modify the emission energy of the        quantum box closest to the mode, and bring it into resonance. In        one variant, the same effect can be achieved by applying a        magnetic field. This approach imposes working within a very        narrow range of growth parameters for the quantum boxes plane,        in order to obtain a low density. For example, in the case of        the InAs/GaAs system, the quantity of InAs deposited must be        controlled to within less than 0.03 nm, which may be impossible        to achieve. In practice, the quantity of InAs at the surface of        epitaxied samples must be varied gradually in order to find a        useful area in which the surface density of quantum boxes is        sufficiently small. Characterisation and tests of structures        become much more difficult, and the efficiency of a        manufacturing process based on this approach will be very low.

It has also been proposed to place one or several quantum boxes in amicrocavity to make lasers with a very low threshold current (4). Thisknown structure is very similar to another known structure consisting ofsurface emission lasers with quantum boxes.

SUMMARY

The purpose of this invention is to propose a solid photon sourcestructure that can be made more simply.

This invention can achieve good manufacturing efficiency for thesesources, and lower their manufacturing cost so that they can bemarketed.

This purpose is achieved according to the invention by means of anopto-electronic component capable of emitting light pulses containing asingle photon comprising a resonant optical cavity and a group of photonemitters placed in this optical cavity, a single one of these emittershaving an emission frequency equal to approximately the resonantfrequency of the cavity, characterised in that all emitters have aspectral distribution with a concentration of emitter frequencies at agiven frequency, and in that the cavity is made so that its resonantfrequency is different from this concentration frequency so that thenumber of emitters with an emission frequency corresponding to theresonant frequency of the cavity is close to one.

BRIEF DESCRIPTION OF THE DRAWINGS

Other purposes, characteristics and advantages of the invention willbecome clear after reading the following detailed description withreference to the appended figures in which:

FIG. 1 shows a single photon source comprising a single emitteraccording to prior art;

FIG. 2 is a plot showing the density of cavity modes for the source inFIG. 1;

FIG. 3 shows a micro-column source with several emitters according tothe invention;

FIG. 4 is a plot showing a spectral distribution of emission frequenciesof emitters and a fundamental resonant mode of the cavity in the case ofthe source in FIG. 3;

FIG. 5 is a plot partially showing the spectral distribution of emissionfrequencies of emitters in FIGS. 3 and 4 with respect to the cavity modeof the source;

FIGS. 6 and 7 are plots showing the same elements as in FIGS. 4 and 5,but in the case of a source operating at a wavelength of 1.3 μm and 300K;

FIG. 8 shows a source according to the invention, designed to operateunder electrical impulse excitation;

FIG. 9 shows a source according to the invention, with opticalexcitation by a surface emission laser;

FIGS. 10 and 11 each show a source similar to that in FIG. 3, opticallypumped for FIG. 10, and electrically pumped for FIG. 11, and in bothcases coupled to an optical fibre for collection of the emitted photons;

FIG. 12 shows a source according to the invention, made from amicro-disk based on GaAs and GaAlAs;

FIG. 13 is a plot showing a distribution of the density of quantum boxmodes compared with a cavity mode in the case of the micro-disk shown inFIG. 12;

FIG. 14 shows an SiO2/Ta₂O₅ Bragg mirror type source according to theinvention.

DETAILED DESCRIPTION

Characteristically, the invention comprises a three-dimensional opticalmicrocavity 100 in which one or several planes 200 of high density (>100boxes per μm²) quantum boxes 250 are inserted such that there is astrong spectral shift A between the useful mode (fundamental mode) ofthe cavity 100 and the maximum emission peak of all quantum boxes 250namely the concentration point of emission peaks of quantum boxes 250.This peak or concentration point is marked as reference P in FIG. 4. Thefundamental mode of the cavity is marked M. This spectral shift Δ isadjusted such that the spectral density of quantum boxes 250 with mode Menergy is less than 0.5/ΔE, where ΔE is the spectral width of mode M ofthe cavity 100.

This shift is easily and very reproducibly obtained by auto-organizedepitaxial growth of quantum box planes 250 with a high box density.

The density and the mean emission wavelength of these planes do notcritically depend on the quantity of InAs deposited, and are veryuniform over the surface of the sample.

The introduction of a strong spectral shift A between the mean emissionpeak P of the boxes 200 and the useful mode of the cavity 100 on thedifferent devices made can reduce the number of boxes 250 coupled tomode M to an average of 0.5 (or less) quantum boxes on all devices made.A single quantum box can then be put into resonance with this mode byadjusting the temperature.

Similarly, this type of resonance can be set up by applying a magneticfield with a selected value to the structure.

This mean peak or the global emission ray of the plane of boxes 250 maybe placed on the “high energies” side or the “low energies” side of thecavity mode M. However, it is preferable to place it on the “highenergies” side of mode M in order to avoid collecting the emissionoriginating from transitions between excited states of quantum boxes250.

For some families of materials such as InAs/GaAs, the spectraldistribution of quantum boxes 250 can be described by a Gaussiandescription as a very good approximation; it is then easy to calculatethe difference Δ between mean peak P/mode M.

Δ can always be adjusted experimentally if this distribution is not aswell known or if it is not as regular.

We will now describe a few example embodiments in detail.

In the example shown in FIG. 3, the microcavity 100 is in the form of acylindrical column with an elliptical section.

FIG. 3 shows a structure conform with the invention, operating at awavelength of 1 μm, using a GaAS/AlAs micro-column and InGaAs quantumboxes.

More precisely, the cavity is sandwiched between two Bragg mirrors eachof which is composed of an alternating series of GaAs and AlAs quarterwavelength layers.

The eccentricity of the section is sufficient such that the fundamentalmode of this cavity is not degenerated (6) (for example small axis 1 μm,large axis 2 μm).

During growth, a plane of InAs or InGaAs quantum boxes 200 with adensity of 400 boxes per μm² and emitting at close to E₀=1.29 eV at 77K,with a typical spectrum width σ=80 meV, is inserted at the heart of thestructure. The number of boxes in this column is therefore close toN=310, and their distribution as a function of the energy is written asfollows:${{n(E)}{dE}} = {2{N/\sigma}\sqrt{\pi}{\mathbb{e}}\frac{{- 4}\left( {E - E_{0}} \right)^{2}}{\sigma^{2}}{dE}}$

For a mode width ΔE of 1 meV (typical of the state of the art, see ref.(6)), it is found that the shift Δ must be equal to at least 60 meV sothat an average of 0.5 boxes are coupled at mode M.

Therefore, for example, we will calculate the structure of microcavity200 so as to adjust the energy of its fundamental mode to 1.23 eV. Asdescribed previously, in this case this microcavity 200 is made in aknown manner in the form of a micro-column with mirrors made fromGaAs/AlAs.

The structure is placed on a base 300 (Stirling cooler) that will keepits temperature at a set point close to 77K, for which a single quantumbox is coupled to the fundamental mode M. A GaAs substrate 350 is placedbetween the base 300 and the micro-column. The structure is opticallypumped by an impulse laser.

FIGS. 6 and 7 show the results obtained with a structure similar to thatin FIG. 3 but adapted to an emission at a wavelength of 1.3 μm and atambient temperature. The fundamental mode is at 0.954 eV (1.3 μm) at300K; the plane of quantum boxes contains 400 boxes/μm², emits at closeto 0.99 eV at 300K, and has a spectral width of 40 meV. It is easy tocheck that the average number of boxes 250 coupled to mode M is lessthan 0.5.

FIG. 7 more particularly shows a random distribution of boxes aroundmode M.

These FIGS. 6 and 7 show a mode density and an average spectraldistribution of quantum boxes (FIG. 6) such that there is a largedifference between M and P, and an example of a particular embodiment ofthe random distribution of boxes (FIG. 7).

The structure in FIG. 8 corresponds to a structure similar to that inFIG. 3, except that the mirrors of the micro-column are doped and inelectrical contact with the ends of the column by an electrical pulsegenerator, so as to achieve electrical excitation of the system.

In this case, the upper Bragg mirror is composed of alternating layersof p doped GaAs/AlAs and the lower mirror is composed of alternatinglayers of n doped GaAs/AlAs. The layer 100 forming the cavity is made ofGaAs that is not intentionally doped. The micro-column is located on ann doped GaAs substrate.

FIG. 9 shows a structure identical to the structure in FIG. 3, exceptthat it is monolithically integrated into a semiconducting surfaceemission microlaser.

The micro-column forms a single photon source with wavelength λ and themicrolaser forms a source at wavelength λ′, where λ′ is less than λ. Themicrolaser is composed of three layers, in other words one active layer400 sandwiched between two Bragg mirrors 500 and 600, the first at themicro-column end being p doped, and the second at the end opposite tothe micro-column being n doped. There are electrodes in contact with thetwo ends of the microlaser, one of the two electrodes surrounding thebottom of the micro-column.

FIGS. 10 and 11 show a structure identical to the structure in FIG. 3,coupled to an optical fibre 700 for the collection of emitted photonsand electrically pumped (FIG. 11) or optically pumped by an opticalfibre (FIG. 10).

In these two examples, the structure in FIG. 3 with its cooler 300 andits substrate 350 is placed in a box 800 and the collection opticalfibre 700 passes through the box 800 from an upper end of themicro-column.

In the case shown in FIG. 11, the excitation electrodes are placed inthe same way as in FIG. 8.

In the case shown in FIG. 11, an excitation optical fibre 900 passesthrough the box laterally to excite the cavity 100 of the micro-column.

FIG. 12 shows a micro-disk 1 000 based on GaAs and GaAlAs containing aplane of quantum boxes 200 made of InGaAs.

This micro-disk 1000 is placed on an elongated base made of GaAlAs,itself placed on a GaAs substrate.

The thickness of the micro-disk is of the order of 200 nm, and the diskdiameter (1 to 5 μm) is adjusted such that one of its confined modes,mode M in FIG. 13, is sufficiently offset from the mean emission of allthe quantum boxes, in other words the concentration point of thefrequencies of quantum boxes 250, and on average is coupled to a smallnumber of boxes (0.1 to 0.5).

An optical fibre 700 (or a semiconductor wave guide) collects emittedphotons. A pass band filter 750 is used to select photons emitted inmode M from the parasite emission emitted in the other modes. In oneparticular embodiment, this filter 750 may be integrated into the fibreas shown in FIG. 12.

FIG. 14 shows a cylindrical micro-column with an elliptical sectioncomposed of a cavity layer 100 made of GaN or GaAlN containing a planeof InGaN quantum boxes 200, and dielectric Bragg mirrors (for examplebased on SiO2 and Ta2O5) on each side of the layer 100. The lower Braggmirror may also be made based on a stack of GaN and GaAlN layers. In onevariant, the quantum boxes are made of GaN and the cavity layer is madeof GaAlN.

An emission wavelength in the visible range (particularly blue andgreen) or the near ultraviolet can be chosen for this single photonsource, by varying the composition and size of the InGaN or GaN quantumboxes.

In this case, and less essentially, it is possible to:

1) add a system to stabilize and adjust the temperature;

2) add a system to collect single photons (fibre, guide, etc.);

3) add a system for spectral filtering of the parasite emission ofquantum boxes not coupled to the mode;

4) integrate means of optical or electrical impulse pumping of quantumboxes in the microcavity.

There are preferred embodiments, for example corresponding to one of thedifferent variants in the appended figures, in which we preferably adoptthe following, for a particularly easy and reliable embodiment:

quantum boxes made of a material with formula In_(x)Ga_(1-x)As, where xis between 0.5 and 1;

the cavity layer made of Ga_(y)Al_(1-y)As where y is between 0.5 and 1,particularly in the case of quantum boxes according to the previousparagraph;

quantum boxes made of InAs and the cavity layer made of GaAs (aparticularly good choice);

in one variant, the quantum boxes are made of In_(x)Ga_(1-x)N, where xis between 0 and 1.

(1) A. Imamoglu and Y. Yamamoto, Phys. Rev. Lett. 72, 210, 1994; J. Kimet al., Nature, 397, 500, 1999

(2) J. M. Gérard, B. Gayral in<<QUED phenomena and applications ofmicrocavities and photonic crystals>>H. Benisty, J. M. Gérard, J. Rarityand C. Weisbuch publishers, Springer-Verlag, Berlin 1999; J. M. Gérardand B. Gayral, J. Lightwawe Technol. 17, 2089 (1999)

(3) B. Gayral, J. M. Gérard, A. Lemaître, C. Dupuis, L. Manin and J. L.Pelouard, Appl. Phys. Lett. 75, 1908 (1999); J. M. Gérard, B. Sermage,B. Gayral, B. Legrand, E. Costard, V. Thierry-Mieg Phys. Rev. Lett 81,1110 (1998)

(4) M. Yamanishi and Y. Yamamoto, J.J.A.P. 30, L60, 1991; and OliverBenson et al. Phys. Rev. Let. 84, 2513, 2000

(5) J. M. Gérard, B. Gayral in<<CQED phenomena and applications ofmicrocavities and photonic crystals>>H. Benisty, J. M. Gérard, J. Rarityand C. Weisbuch publishers, Springer-Verlag 1999; J. M. Gérard and B.Gayral, J. Lightwawe Technol. 17, 2089 (1999)

(6) B. Gayral et al. Appl. Phys. Lett. 72, 1421, 1998

1. Opto-electronic component capable of emitting light pulses containing a single photon, opto-electronic comprising: a resonant optics cavity (100) and a group of photon emitters (250) placed in this optical cavity (100), a single one of these emitters (250) having an emission frequency equal to approximately the resonant frequency (M) of the cavity (100), wherein all emitters have a spectral distribution with a concentration of frequencies of emitters (250) at a given frequency (P), and wherein the cavity (100) is made so that its resonant frequency (M) is sufficiently different from this concentration frequency (P) so that the number of emitters (250) with an emission frequency corresponding to the resonant frequency (M) of the cavity (100) is close to one.
 2. Component according to claim 1, characterised in that the emitters (250) are generated from one or more semiconductor materials.
 3. Component according to claim 2, wherein the emitters (250) are semiconducting quantum boxes.
 4. Component according to claim 3, wherein the quantum boxes are made of In_(x)Ga_(1-x)As, where x is between 0.5 and
 1. 5. Component according to claim 4, wherein the cavity is composed of a Ga_(y)Al_(1-y)As layer, where y is between 0.5 and
 1. 6. Component according to claims 4 and 5 in combination, wherein x=1 and y=1.
 7. Component according to claim 1, wherein component's mode spectral density of the emitters (250) at the frequency of the cavity mode (M) is close to 0.5 emitters for the width of the cavity mode (M).
 8. Component according to claim 1, wherein the component includes means of adjusting emission frequencies of emitters such that the frequency of a single emitter corresponds to the resonant frequency of the cavity.
 9. Component according to claim 1, wherein the component includes a means (750) of filtering the photon(s) emitted at the frequency of the cavity mode (M) from photons of emitters not coupled to the cavity mode.
 10. Component according to claim 1, wherein the component includes optical or electrical impulse pumping means (400, 500, 600, 900) of the emitters (250).
 11. Component according to claim 1, wherein the component includes means (700) of collecting the emitted single photon pulses.
 12. Component according to any one of claims 1 to 11 in combination with claim 3, wherein the quantum boxes are made of In_(x)Ga_(1-x)N, where x is between 0 and
 1. 